Conventional electrodes for monolithic micromechanical transducers consist of an electrode structure on a separate substrate or doped regions within the same silicon substrate that houses the transducer, at least partly bounded by a PN junction. In a buried electrode structure that comprises a region of P+ semiconductor material in an N-type silicon substrate, over which an N-type layer is grown and into which the transducer is formed.
Buried electrodes have a large parasitic capacitance to the substrate. Furthermore, the parasitic capacitance is voltage dependent, creating nonlinear errors. Such electrodes also exhibit a small leakage current. The maximum voltage that a buried electrode can withstand is the reverse breakdown voltage of the PN junction formed by it, typically about 40 volts. Buried electrodes must be disposed parallel to each other within the same plane so that they cannot be oriented in orthogonal relationships with respect to each other, as required by some applications.
Examples of prior systems are illustrated in commonly assigned copending U.S. application Ser. No. 07/314,453, filed Feb. 22, 1989 (CSDL-099XX), and U.S. Pat. No. 4,044,305.